This invention relates generally to 3-dimensional waveguide resonators and, more particularly, to waveguide resonators suitable for applications in the microwave bands and beyond. High-Q resonators are critical components of voltage controlled oscillators (VCOs) and filters, which are widely used in communication systems. There is an ongoing trend in communication systems to utilize higher frequencies. Higher frequencies are not only a less congested area of the radio frequency (RF) spectrum, but also provide technical advantages such as increased bandwidth and increased reliability for military and commercial applications.
A common measure of the performance of a resonator is its quality factor, or Q factor. Basically, the Q factor is a measure of the sharpness of resonance of a resonator. A device with a high Q factor has a sharp, well defined resonance at certain frequency. The Q factor may also be defined as the ratio of the stored energy to the dissipated energy in one cycle. The Q factor is then determined by the cavity loss of the cavity. It is a measure for the damping of waveguide modes. The higher the value of Q, the less loss or damping effect. Unfortunately, it becomes increasingly difficult to design high-Q resonators as the frequency increases. At millimeter wave frequencies, for example, there are a number of important applications of resonators, but conventional implementations using dielectric resonators (DR) or coaxial ceramic resonators (CCR) become impractical due to manufacturing limitations. Generally speaking, a millimeter wave has a wavelength in the range of 1 mm to 0.1 mm and a frequency in the range of 300 gigahertz (GHz) to 3,000 GHz.
The conventional DR and CCR approaches have several disadvantages. The first is lack of tunability. Most existing resonators are not electronically tunable. Frequency tuning generally involves mechanical tuning of the resonator structures, which is tedious, costly and challenging.
A second disadvantage of conventional resonator approaches is their difficulty of manufacturability and ability to be manufactured repeatably. The dimensions of resonators become too small to be practical for DRs and CCRs at frequencies above 40 GHz. Most existing high-Q resonators are implemented “off-chip,” that is to say separately from other related components. When connecting to oscillators or to other MMICs (monolithic microwave integrated circuits), ribbons or bond wires are used. These not only introduce parasitic impedance effects, but also greatly reduce the repeatability of the overall circuit's performance and tunability.
Prior to the present invention, most existing monolithically integrated resonators were of a planar type. Planar resonators inherently have a relatively low Q factor, resulting in poor phase noise for a VCO of which such a resonator is a part, and in compromised insertion loss and rejection for filter applications of the resonators.
Yet another disadvantage of resonators of the prior art is their overall high cost. Scaling DRs and CCRs down in size for higher frequencies of operation is not only technically difficult, but it leads inherently to higher manufacturing cost.
Accordingly, there is a real need for a new approach to resonator construction that lends itself more readily to scaling to increasingly high frequencies, that is electronically tunable and, ideally, that still maintains a high Q factor. The present invention meets these requirements, as will become apparent from the following summary.